In the past, there has been known an air pollution control system that treats a flue gas discharged from a boiler installed in a thermal power facility or the like. The air pollution control system includes a denitration device that removes a nitrogen oxide from the flue gas discharged from the boiler, an air heater that recovers the heat of the flue gas having passed through the denitration device, a precipitator that removes soot and dust contained in the flue gas from which heat has been recovered, and a desulfurization device that removes a sulfur oxide contained in the flue gas from which soot and dust have been removed. A wet desulfurization device, which removes a sulfur oxide contained in a flue gas by bringing a limestone absorbent or the like into gas-liquid contact with the flue gas, is generally used as the desulfurization device.
A large amount of various toxic substances, such as ions such as chlorine ions and ammonium ions and mercury, are contained in waste water (hereinafter, referred to as “desulfurization waste water”) discharged from the wet desulfurization device. For this reason, it is necessary to remove these toxic substances from the desulfurization waste water before the desulfurization waste water is discharged to the outside of a system. However, since treatment for removing these various toxic substances contained in the desulfurization waste water is complicated, there is a problem in that treatment costs are high. Accordingly, a method, which reuses desulfurization waste water in a system without discharging desulfurization waste water to the outside of a system, has been proposed to reduce the treatment costs for desulfurization waste water. For example, Patent Literatures 1 and 2 disclose an air pollution control apparatus including a facility that branches a flue gas from a flue gas duct of a main line to which a denitration device, an air heater, a precipitator, and a desulfurization device are connected, gasifies desulfurization waste water by spraying the desulfurization waste water, and is separately installed. The apparatus introduces a part of a flue gas to this facility from the flue gas duct of the main line, precipitates toxic substances by spraying the desulfurization waste water into the flue gas in the facility in order to evaporate the desulfurization waste water, and then returns the flue gas to the flue gas duct of the main line (Patent Literatures 1 and 2).